The present invention relates to the field of magnetic resonance imaging (MRI) systems and, more particularly, concerns radio frequency (RF) coils for use in such systems.
In MRI systems or nuclear magnetic resonance (NMR) systems, radio frequency signals are provided in the form of circularly polarized or rotating magnetic fields having an axis of rotation aligned with a main magnetic field. An RF field is then applied in the region being examined in a direction orthogonal to the static field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Receiving coils intercept the radio frequency magnetic field generated by the subject under investigation in the presence of the main magnetic field in order to provide an image of the subject. Typically, such RF coils are either surface-type coils or volume-type coils, depending upon the particular application. Normally, separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes.
Conventional MRI systems have a number of artifact problems. For example, aliasing of unwanted signals into the resonance object image is a common problem in MRI applications. A particular form of artifact, sometimes referred to as an aliasing artifact, can occur in the either the frequency direction or the phase direction within MRI systems. In this type of artifact, an area of anatomy that is at least partially within the excitation field of the body coil has a local Larmor frequency identical to a pixel within the imaging field of view. This phenomenon typically originates from areas outside the field of view, but causes artifacts inside the image. It often arises as a result of the non-linearity of the gradient fields and/or non-homogeneity of the DC magnetic fields.
Accordingly, to reduce the occurrences of unwanted artifacts, there exists a need for MRI systems having improved linearity of gradient fields and homogeneity of DC magnetic fields in RF transmit coils with zero sensitivity outside the imaging field of view.
In the present invention, the aforementioned problem is solved through the provision of a novel transmit coil or array of coils that is sensitive in the imaging volume, but radically drops off in sensitivity outside of the image field of view. An advantage of the present invention is that it does not excite the spin system in areas where the aliasing artifact originates.
In particular, the present invention provides a radio frequency (RF) coil system for magnetic resonance imaging/analysis comprising a primary coil element having a plurality of axial conductors spaced to form a generally tubular structure having two ends and defining a coil volume, and a first pair of spoiler coils. The spoiler coils each comprise a plurality of axial conductors spaced to form a generally tubular structure and define a coil volume. Each of the spoiler coils is positioned adjacent to and slightly overlapping an end of the primary coil. Each of the primary and spoiler coils is also adapted to carry an RF signal, wherein the signal in the rungs of the spoiler coils is 180 degrees out of phase with the signal in the rungs of the primary coil.
In another aspect of the invention, an additional pair of spoiler coils are added adjacent to and outside of the first pair of spoiler coils with the signal carried in the additional pair of spoiler coils being 180 degrees out of phase with the signal carried in the first pair of spoiler coils.
In a further aspect of the invention, a RF apparatus for use in a nuclear magnetic resonance (NMR) system is provided. The RF coil has a generally tubular structure defined by an inner wall and an outer wall, the inner wall defining an imaging volume. The RF coil also comprises a plurality of discrete electrically conductive members positioned between the inner wall and the outer wall which are equally circumferentially spaced around the tubular structure so as to form opposing pairs of conductive members. Each of the conductive members comprises a conductive loop having a primary coil section and a spoiler coil section at each end of the primary coil section configured such that a current flow of a signal on the loop in a cross-over region between each of the spoiler coils and the primary coil is in opposite directions.
An advantage of the present invention is that it provides homogeneity of the magnetic field throughout the image field of view, but radically drops off in sensitivity outside the field of view to reduce the likelihood of aliasing artifacts. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.